2 Things.

1. NATO promised USSR president Gorbachev in 1990 not to expand eastwards. Now NATO expansion has reached Russia's doorstep, betraying the promise. So, nobody talks about promise anymore since it is NATO who broke the promise first. Trust is already broken.

2. Nukes in Ukraine at the time did not belong to them. The command codes are in Kremlin and belong to the Russians. Even if Ukraine kept the nukes at the time, they couldn't use it.
If US can justify the use of nukes in Japan during WW2, I don't see how Russia cannot justify the use of nukes in Ukraine since Ukraine strike their nuke delivery means first. Since Russia already gotten hit with so much sanctions, Putin has nothing to lose if he use nukes in Ukraine.